As a conventional fuel cell power generation system, there has been known a system which has the structure shown in FIG. 19 (for example, see pages 3-4, FIG. 1 and the like of Japanese Patent Laid-open Hei6 (1994)-68894 (referred as document 1)).
As shown in FIG. 19, the conventional fuel cell power generation system includes a raw material supply source 100 which supplies a raw material gas, a desulfurizing unit 101 which removes a sulfur component from the raw material gas, a reformer 102 which generates a hydrogen-rich gas from a mixed gas of a natural gas from which a sulfur component is removed and a water vapor by making use of a water vapor reforming reaction, and a carbon monoxide decreasing unit 103 which decreases carbon monoxide in the hydrogen-rich gas. Further, the fuel cell power generation system includes a fuel cell 105, wherein the hydrogen-rich gas with a decreased quantity of carbon monoxide which is sent to a fuel electrode and compressed air which is fed to an air electrode electrochemically react each other thus producing electricity, water and heat. A discharged fuel gas which is discharged from the fuel cell 105 is supplied to a combustion burner 106 which heats the reformer 102 and the discharged fuel gas is used to heat the reformer 102. The above-mentioned desulfurizing unit 101, the reformer 102 and the carbon monoxide reducing unit 103 mentioned above constitute a fuel treatment device 104.
Further, between the fuel treatment device 104 and the raw material supply source 100, a raw material gas supply passage 107 is provided and a raw material gas shut-off valve 108 is provided on the raw material gas supply passage 107. Further, between the fuel treatment device 104 and the fuel cell 105, a fuel-gas supply passage 109 is provided and a fuel-gas shut-off valve 110 is provided on the fuel-gas supply passage 109. Still further, between the fuel cell 105 and the combustion burner 106, a fuel-gas discharge passage 111 is provided and a fuel-cell outlet shut-off valve 112 is provided.
Further, a fuel cell bypass passage 113 which connects the fuel-gas supply passage 109 between the fuel-gas shut-off valve 110 and the fuel treatment device 104, and the fuel-gas discharge passage 111 between the fuel-cell outlet shut-off valve 112 and the combustion burner 106 is provided and a bypass passage shut-off valve 114 is provided on the fuel cell bypass passage 113.
In this manner, a plurality of shut-off valves are provided upstream and downstream of the fuel treatment device 104.
When the fuel cell power generation system is stopped, the raw material gas shut-off valve 108 is closed so as to stop the supply of the raw material gas, thereafter the respective shut-off valves 110, 112, 114 are closed.
However, in the conventional fuel cell power generation system, these solenoid valves are closed when the system operation is stopped, so that a closed passage is formed in flow passages including the fuel treatment device 104. On the other hand, although the inside of the fuel treatment device 104 assumes a temperature of 600° C. or more during the operation of the system, after operation of the system is stopped, the temperature is lowered over a lapse of time. Further, the water vapor which is contained in a fuel gas produced by the fuel treatment device 104 is condensed along with the lowering of the temperature whereby the lowering of pressure occurs in the closed passage.
From the above, in the fuel cell power generation system which performs starting and stopping, there has been a drawback that the closed passage of the flow passage assumes a negative pressure due to the lowering of the pressure after the operation of the fuel cell power generation system is stopped. This generation of negative pressure inside the closed passage becomes a factor which causes a system failure attributed to seizure of a solenoid valve or the deterioration of performance of a catalyst in the inside of the fuel treatment device 104 attributed to the inflow of air from the outside.
Accordingly, to overcome such drawbacks, the following fuel cell power generation systems have been proposed (for example, see the embodiment 1, paragraphs [0021], FIG. 2 and FIG. 3 of Japanese Patent Laid-open Heill (1999)-191426 (referred to as document 2), see paragraphs [0017] and FIG. 1 of Japanese Patent Laid-open 2000-95504 (referred to as document 3).
More specifically, the fuel cell power generation system described in the document 2 aims at the prevention of a negative pressure when the operation of the system is set in a stop mode. That is, the document 2 discloses the constitution in which a deoxygen device for supplying deoxidized air is provided upstream of a fuel reforming device, between a fuel reformer and a CO modifying unit or between a fuel reformer and a fuel cell.
Further, a reforming device described in document 3 discloses a constitution in which even when the operation of the reforming device is stopped and temperatures of respective reaction units are lowered and hence, gases in the inside of the respective reaction units are contracted, pressures in the inside of the respective reaction units are maintained at constant pressures by supplying a raw material gas into the reforming reaction units.
However, in the method described in document 2, the deoxidized air, that is, an incombustible gas which contains a large quantity of nitrogen is supplied to the inside of the fuel reforming device. Accordingly, at the next start of operation, at the time of performing the combustion treatment of a large quantity of residual fuel gas contained in the fuel reforming device, there arises a drawback that a combustion state becomes unstable. Further, the method requires a device such as a deoxidizing device or the like and hence, there arises a drawback that the constitution becomes complicated and increases the cost.
Further, as described in document 2, it may be possible to use a raw material gas or the like which is used in generation of power in place of an incombustible gas. However, the use of such a raw material gas may induce the precipitation of carbon in the reformer. Accordingly, as described in document 3, there arises a drawback that it is necessary to provide a wasteful step in terms of energy that a raw material and water are supplied until the temperature is lowered to a value equal to or below a carbon precipitation temperature. Further, to eliminate such wasteful use of energy, it may be possible to adopt a method in which the fuel reforming device is formed in a hermetically closed system and the fuel reforming device is subject to natural cooling. However, in this case, the fuel reforming device may not be able to withstand the generation of a level of negative pressure when the temperature is lowered to a temperature (approximately 300° C.) which lowers the possibility of carbon precipitation from a reformer temperature (approximately 600° C.) at which an usual operation is performed and hence, the device may break down.